SOUTH Pole

SOUTH Pole

NORTHPole

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MAGNETMAGNETIC FIELD

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MAGNETIC CIRCUIT, ELECTROMAGNETISM

AND ELECTROMAGNETICINDUCTION

The end of lesson, students The end of lesson, students should be ;should be ;

Understand magnetism Understand the composite series magnetic

circuit Understand the electrical and magnetic

quantities Understand hysteresis Understand electromagnetism Determine the magnetic field direction. Understand electromagnetic induction

iNTRoDUctIONiNTRoDUctION

MAGNET isthe material that have two poles NORTH and SOUTH

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NORTHPole


MAGNET can be define as

Material that can attract piece of iron or metal

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iNTRoDUctIONiNTRoDUctIONMATERIAL that ATTRACTED by the MAGNET is known as

MAGNETIC SUBSTANCES

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iNTRoDUctIONiNTRoDUctIONThe ABILITY to ATTRACT the MAGNETIC SUBSTANCES is known as

MAGNETISM

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MAGNETIC FIELD is the force around the MAGNET which can attract any MAGNETIC MATERIAL around it.

FLUX MAGNET is the line around the MAGNET bar which form MAGNETIC FIELD.

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TYpEs of MAGNETTYpEs of MAGNET

There are 2 types of MAGNET

PURE MAGNETMANUFACTURE MAGNET

PURE MAGNET

Known as MAGNET STONE

The stone ORIGINALY have the NATURAL MAGNETIC

Basically the stone is found in the form of IRON ORE

MANUFACTURE MAGNET

There are 2 types of MANUFACTURE MAGNET

PERMANENT MAGNETTEMPORARY MAGNET

PERMANENT MAGNET

The ABILITY of the MAGNET to kept its MAGNETISM

The basic shape of PERMANENT MAGNET

U shapehorseshoe RODCylinderBAR

PERMANENT MAGNET

U shapeHorseshoe Rod

Cylinder

Bar

Permanent magnet can be obtained by:naturally or magnetic induction ( metal rub against natural magnet)

placing a magnet into the coil and then supplied with a high electrical current.

PERMANENT MAGNET

Permanent magnet used in small devices such as:

PERMANENT MAGNET

speakers metercompass

TEMPORARY MAGNETTEMPORARY MAGNET

BECOME MAGNET only when there is CURRENT SUPPLY to the metal

It has magnetic properties when subjected to magnetic force and it will be lost when power is removed.

TEMPORARY MAGNETTEMPORARY MAGNETExample :

relayelectric bells

Magnetic flux lines have direction and pole.

The direction of movement outside of the magnetic field lines is from north to south.

CHARACTERISTICS OF MAGNETIC FORCE LINES (FLUX).

The strongest magnetic field are at the magnetic poles .

DIFFERENT POLES ATTRACT each other

SAME MAGNETIC POLES will REPEL each other

SN SN

SN S N


FLUX form a complete loop and never intersect with each other.

FLUX will try to form a loop as small as possible.

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MAGNETIC QUANTITY MAGNETIC QUANTITY CHARACTERISTICSCHARACTERISTICS

Magnetic Flux Magnetic flux is the amount of

magnetic field produced by a magnetic source.

The symbol for magnetic flux is . The unit for magnetic flux is the

weber, Wb.

MAGNETIC QUANTITYMAGNETIC QUANTITYCHARACTERISTICSCHARACTERISTICS

Magnet Flux density The symbol for magnetic flux

density is B. The unit is tesla, T the unit for area A is m2 where

1 T = 1 Wb/m.


Magnet Flux density Magnetic flux density is the

amount of flux passing through a defined area that is perpendicular to the direction of flux


Magnetic flux density = area

flux magnetic

A

ΦB Tesla


Example 3

A magnetic pole face has rectangular section having dimensions 200mm by 100mm. If the total flux emerging from the pole is 150Wb, calculate the flux density.

A

ΦB

Area, A

Flux, Φ

B?


Solution 3Magnetic flux, = 150 Wb = 150 x 10-6 WbCross sectional area, A = 200mm x 100mm

= 20 000 x 10-6 m2

Flux density,

= 7.5 mT

6

6

1020000

10150

A

ΦB

MAGNETOMOTIVE FORCE (MMF)MAGNETOMOTIVE FORCE (MMF) The force which creates the magnetic flux in a

magnetic circuit is called magnetomotive force (mmf)

- The mmf is produced when a current passes through a coil of wire. The mmf is the product of the number of turns(N) and current (I) through the coil.

Unit = Ampere Turns (A.T)

Formula , Fm = N x I

MAGNETIC FIELD STRENGTH,H MAGNETIC FIELD STRENGTH,H (MAGNETISING FORCE)(MAGNETISING FORCE)

Defined as magnetomotive force, Fm per metre length of measurement being ampere-turn per metre.

Current

l

NI

l

FH m

magnetomotive force

number of turns

average length of magnetic circuit


Example 1

A current of 500mA is passed through a 600 turn coil wound of a toroid of mean diameter 10cm. Calculate the magnetic field strength.

l

NI

l

FH m

Current, I

Turn, N

Diameter, d

H?


Solution 1I = 0.5AN= 600

l = x 10 x 10-2m

mATH

H

metreampereturnl

NIH

/81.9543142.0

5.0600

/


Example 2

An iron ring has a cross-sectional area of 400 mm2. The coil resistance is 474 Ω and the supply voltage is 240 V and a mean diameter of 25 cm. it is wound with 500 turns. Calculate the magnetic field strength, H


Solution 2I = V/ R = 240 / 474 = 0.506 Al = π D = π (25 x10-2) = 0.7854 m H=

H=

H= 322.13 AT/m

l

NI

7854.0

506.0500

PERMEABILITYPERMEABILITYFor air, or any other non-

magnetic medium, the ratio of magnetic flux density to magnetic field strength is constant ,

This constant is called the permeability of free space and is equal to 4 x 10-7 H/m.

H

B

µ0

PERMEABILITYPERMEABILITY

For any other non-magnetic medium, the ratio

For all media other than free space

r

rH

B 0


r is the relative permeability and is defined as

r varies with the type of magnetic material.

in vacuumdensity flux

materialin density flux r


r for a vacuum is 1 is called the absolute permeability.

The approximate range of values of relative permeability r for some common magnetic materials are :

Cast iron r = 100 – 250Mild steel r = 200 – 800Cast steelr = 300 – 900


Example 4

A flux density of 1.2 T is produced in a piece of cast steel by a magnetizing force of 1250 A/m. Find the relative permeability of the steel under these conditions.

HB r0

Flux density, B

H

µr?


Solution 4

HB r0

764

)1250)(104(

2.17

0

H

Br

RELUCTANCERELUCTANCE

Reluctance,S is the magnetic resistance of a magnetic circuit to presence of magnetic flux.

Reluctance,

The unit for reluctance is 1/H or H-1 or A-T/Wb

AAHBBA

HlFS

r

m

0)/(

RELUCTANCERELUCTANCEExample 5

Determine the reluctance of a piece of metal of length 150mm and cross sectional area is 1800mm2when the relative permeability is 4 000. Find also the absolute permeability of the metal.

S?

Length, l µrµ?

RELUCTANCERELUCTANCESolution 5Reluctance,

= = 16 579 H-1

Absolute permeability, =

AS

r0

)101800)(4000)(104(

1015067

3

r 0)4000)(104( 7

= 5.027 x 10-3 H/m

ELECTROMAGNETELECTROMAGNET Is a magnetic iron core

produced when the current flowing through the coil.

Thus, the magnetic field can be produced when there is a current flow through a conductor.

The direction of the magnetic field can be determined using the method:

1. Right Hand Grip Rules 2. Maxwell's screw Law. 3. Compass

Three rules may be used to indicate the direction of the current and the flux produced by current carrying conductor.

Right Hand Grip RuleRight Hand Grip Rule

is a physics principle applied to electric current passing through a solenoid, resulting in a magnetic field.

Right Hand Grip RuleRight Hand Grip RuleWhen you wrap your right hand

around the solenoid

your fingers in the direction of the conventional current

your thumb points in the direction of the magnetic north pole

Right Hand Grip RuleRight Hand Grip RuleIt can also be applied to

electricity passing through a straight wire

the thumb points in the direction of the conventional current (from +ve to -ve)

the fingers point in the direction of the magnetic lines of flux.

Another way to determine the direction of the flux and current in a conductor is to use Maxwell's screw rule.

MAXWELL’S SCREW LAW

a right-handed screw is turned so that it moves forward in the same direction as the current, its direction of rotation will give the direction of the magnetic field.

MAXWELL’S SCREW LAW

Electromagnetic Effect Electromagnetic Effect Direction of Current

going INside Solenoid

Direction of Magnetic Flux around Solenoid

Direction of Current going OUTside

Solenoid


Right Hand Grip Rule

Electromagnetic Effect Direction of Current

going INside Solenoid


Direction of Current going OUTside

Solenoid


Same Direction Different Direction

Maxwell Screw Law

Electromagnetic Effect Electromagnetic Effect

Factors that influence the strength of the magnetic field of a solenoid

The number of turns The value of current flow Types of conductors to produce

coil The thickness of the conductor

ELECTROMAGNETIC INDUCTION ELECTROMAGNETIC INDUCTION Definition : When a conductor is moved

across a magnetic field so as to cut through the flux, an electromagnetic force (emf) is produced in the conductor.

This effect is known as electromagnetic induction.

The effect of electromagnetic induction will cause induced current.

ELECTROMAGNETIC INDUCTION ELECTROMAGNETIC INDUCTION

2 laws of electromagnetic induction:i. Faraday’s lawii.Lenz’z Law

Faraday’s lawFaraday’s lawIt is a relative movement of the magnetic

flux and the conductor then causes an emf and thus the current to be induced in the conductor.

Induced emf on the conductor could be produced by 2 methods flux cuts conductor or conductor cuts flux.

Faraday’s lawFaraday’s lawFaraday’s First Law : Flux cuts conductor

When the magnet is moved towards the coil, a deflection is noted on the galvanometer showing that a current has been produced in the coil.

Faraday’s lawFaraday’s lawFaraday’s Second Law :Conductor cuts flux

When the conductor is moved through a magnetic field . An emf is induced in the conductor and thus a source of emf is created between the ends of the conductor.

Faraday’s lawFaraday’s lawThis induced electromagnetic field is given by E = Blv

volts

B=flux density, Tl =length of the conductor in the magnetic field, mv=conductor velocity, m/s

If the conductor moves at the angle to the magnetic field, then

E = Blv sin volts

Faraday’s lawFaraday’s lawExample

A conductor 300mm long moves at a uniform speed of 4m/s at right-angles to a uniform magnetic field of flux density 1.25T. Determine the current flowing in the conductor when :

a. its ends are open-circuitedb. its ends are connected to a load of 20

resistance.

Faraday’s lawFaraday’s lawSolution

a. If the ends of the conductor are open circuited no current will flow .

Faraday’s lawFaraday’s lawSolution

b. E.m.f. can only produce a current if there is a closed circuit. When a conductor moves in a magnetic field it will have an e.m.f. induced.Induced e.m.f. , E = Blv

=(1.25)(0.3)(4) = 1.5 v

From Ohm’s law

mAI

I

R

EI

7520

5.1

Lenz’z lawLenz’z lawThe direction of an induced emf is always

such that it tends to set up a current opposing the motion or the change of flux responsible for inducing that emf

FormulaFormula

AS

r0

l

NI

l

FH m

A

ΦB

MAGNETIC FIELD STRENGTH

RELUCTANCE

MAGNETIC FLUX DENSITY

PERMEABILITY H

Br0

AAHBBA

HlFS

r

m

0)/(

MAGNETOMOTIVE FORCE (MMFMAGNETOMOTIVE FORCE (MMF), Fm = N x I

Composite magnetic circuitA series magnetic circuit that has parts of different dimensions and material is called composite magnetic circuit.Each part will have its own reluctance. Total reluctance is equal to the sum of reluctances of individual parts.

Total reluctanceTotal reluctance

Comparison between magnetic Comparison between magnetic and electric circuitand electric circuit

Similarities & dissimilarities between magnetic circuit and electric circuit

Hysterisis and hysterisis lossHysterisis and hysterisis loss

Figure 7.6

SOUTH Pole

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